1. Field of the Invention
The present invention relates to improved multilayer interconnect systems having conductive circuit layers adhering to and sandwiched between polyimide dielectric layers, and a process for fabricating same.
2. Description of Related Art
High-performance mainframe computers require a very high density interconnect system to take advantage of high speeds of the integrated circuits currently available. These interconnect systems are normally fabricated using multilayer thin film technology to produce a plurality of conductor circuit planes or layers containing a circuit pattern of conductive metal lines, e.g. copper metal lines, sandwiched between and adhered to a plurality of dielectric planes or layers which can be ceramic layers, layers of polyimide polymer or combinations of such layers.
Multilayer interconnect systems which utilize polyimide polymers as the dielectric insulating layer are generally prepared by well known processes. A thin film of conductive metal, e.g. copper, is deposited on the surface of a substrate layer which may be a polymeric film, e.g. polyimide, or a dielectric ceramic. The metal layer is then coated with a thin film of photoresist, followed by exposure of the resist film to a patterned source of light or energy. Subsequent development of the resist leaves exposed metal and resist-masked metal lines which will form the circuit pattern. Exposed metal is then selectively etched away using a suitable etchant, and the resist mask is subsequently removed by known techniques leaving a circuit pattern of conductive metal lines adhered to the substrate.
Laminar structures are prepared by coating the surface of the circuit structure prepared as above with a polyimide precursor material, e.g. a polyamic acid, to form a thin overcoating film layer which contacts both the exposed substrate and the circuit pattern. The precursor is then cured by heating the assembly at a temperature up to 450.degree. C. which results in the formation of a dielectric layer comprising the polyimide condensation product. The above described process is then repeated one or more times by forming additional layers of conductive metal on the surface of the polyimide polymer dielectric layer, patterning and etching this layer as described above, and further overcoating the patterned conductive layer with polyimide precursor material, curing the precursor material, and so forth.
The use of polyimide polymer as the dielectric layer in such constructions offers the advantages of ease of fabrication and the provision of a smooth dielectric surface which facilitates the development of a multi layer, dense laminar package having many alternating layers of dielectric and metal conductors stacked one on top of the other.
One problem with respect to such a package construction is that some conductive metals, such as copper, chemically interact when in contact with polyamic acids resulting in the development of a copper complex with the free carboxylic acid group present on the polyamic acid chain, likely catalyzed by the generation of water of hydration during the condensation reaction from the polyamic acid stage to the cured polyimide stage. This interaction produces a phenomenon on the interfacial surfaces between the copper conductors and polyimide which significantly reduces or destroys the electrical properties of the package, a particularly serious problem where the interconnect system is of very high conductor density. Corrosion of the copper conductors can result in electrical shorts and/or delamination of the sandwich structure.
One approach to the solution of this problem is to sputter a capping layer of a relatively inert metal, such as chromium, over the surface of the copper line interconnects during fabrication of the laminar interconnect package, which capping layer serves to chemically insulate the copper from contact with the later-applied polyamic acid precursor. However, sputter application of a chromium layer is non-selective and therefore requires additional lengthy patterning and etching steps which add significantly to cost.
Another approach toward the solution of this problem is disclosed in U.S. Pat. No. 4,770,899, which teaches electroless deposition of an inert capping metal layer selectively on the surfaces of copper conductors. The capping metal is preferably nickel or cobalt and serves to chemically insulate the copper and the polyamic acid precursor. One problem with the approach is that such electrolessly deposited metals exhibit significantly less adhesion with respect to polyimide films formed on their surfaces than do metals such as chromium, thereby leading to a prospect of laminar adhesion failures at the interface of the copper conductor and polyimide layers.
Accordingly, it is a primary object of this invention to prepare a multilayer laminar interconnect package wherein conductive metal line components of conductive layers are chemically insulated from overlying polyimide dielectric layers and which exhibit enhanced adhesion at the interface of the conductors and polyimide dielectric layers.